Rework word_similarity documentation, make it close to actual algorithm.

Rework word_similarity documentation, make it close to actual algorithm.

word_similarity before claimed as returning similarity of closest word in
string, but, actually it returns similarity of substring. Also fix mistyped
comments.

Author: Alexander Korotkov
Review by: David Steele, Liudmila Mantrova
Discussionis:
https://www.postgresql.org/message-id/flat/CY4PR17MB13207ED8310F847CF117EED0D85A0@CY4PR17MB1320.namprd17.prod.outlook.com
https://www.postgresql.org/message-id/flat/f43b242d-000c-f4c8-cb8b-d37e9752cd93%40postgrespro.ru